The free radical superoxide anion is a product of normal cellular metabolism, produced mainly in mitochondria because of incomplete reduction of oxygen. Superoxide is harmful because it reduces iron-III to iron-II, and interacts with nitric oxide radical forming the strong oxidant, peroxynitrite. Superoxide is scavenged by superoxide dismutases forming hydrogen peroxide. Hydrogen peroxide can interact with transition metal ions to form the hydroxyl radical, the strongest oxidant formed in biological systems. Destructive actions of the hydroxyl radical include alterations of DNA and initiation of chain reactions of lipid peroxidations. Preventing these actions from occurring are glutathione peroxidase (GPx) enzymes that scavenge inorganic and organic peroxides. Regulation of GPx gene expression is complex since they are selenoproteins, requiring, for example, specific selenocysteinyl t-RNA and translational machinery to enable recognition of UGA, a stop codon, as a selenocysteine codon. Action of GPx also requires reduced- glutathione (GSH) as the electron donor. Regulation of cellular levels of GSH is also complex. Cells that produce high levels of superoxide, such as neurons and oligodendrocytes, tend to have low levels of both GSH and GPx. My thesis is that slight increases in Gpx and/or GSH ought to delay the onset of a variety of neurodegenerative diseases.